Stop single file and --files-from operations iterating through the source bucket.

This works by making sure directory listings that use a filter only
iterate the files provided in the filter (if any).

Single file copies now don't iterate the source or destination
buckets.

Note that this could potentially slow down very long `--files-from`
lists - this is easy to fix (with another flag probably) if it causes
anyone a problem.

Fixes #610
Fixes #769

fs/filter.go

@@ -94,8 +94,8 @@ func (rs *rules) len() int {
 	return len(rs.rules)
 }
 
-// filesMap describes the map of files to transfer
-type filesMap map[string]struct{}
+// FilesMap describes the map of files to transfer
+type FilesMap map[string]struct{}
 
 // Filter describes any filtering in operation
 type Filter struct {
@@ -106,8 +106,8 @@ type Filter struct {
 	ModTimeTo      time.Time
 	fileRules      rules
 	dirRules       rules
-	files          filesMap // files if filesFrom
-	dirs           filesMap // dirs from filesFrom
+	files          FilesMap // files if filesFrom
+	dirs           FilesMap // dirs from filesFrom
 }
 
 // We use time conventions
@@ -313,8 +313,8 @@ func (f *Filter) AddRule(rule string) error {
 // AddFile adds a single file to the files from list
 func (f *Filter) AddFile(file string) error {
 	if f.files == nil {
-		f.files = make(filesMap)
-		f.dirs = make(filesMap)
+		f.files = make(FilesMap)
+		f.dirs = make(FilesMap)
 	}
 	file = strings.Trim(file, "/")
 	f.files[file] = struct{}{}
@@ -332,6 +332,13 @@ func (f *Filter) AddFile(file string) error {
 	return nil
 }
 
+// Files returns all the files from the `--files-from` list
+//
+// It may be nil if the list is empty
+func (f *Filter) Files() FilesMap {
+	return f.files
+}
+
 // Clear clears all the filter rules
 func (f *Filter) Clear() {
 	f.fileRules.clear()